Key points
- The camera decides pass or fail; the PLC tracks the product and fires the reject.
- Digital I/O is simple and direct; an industrial network carries more data. Which protocols are available depends on the camera or controller model.
- A fixed time delay suits constant-speed lines; an encoder keeps the reject on the right product when speed changes.
- A reject is only finished when a sensor confirms the product left the line.
- Behaviour on faults is designed, written down and tested. It is general control practice, not a certified safety function.
The signal chain at a glance
A vision station does not reject anything by itself. It decides whether a product passes; the line's PLC decides what happens next. Between the two sits a chain of signals: a trigger, an exposure, a decision, a result, tracking, the reject and a confirmation. Each link can fail, and most problems on real lines come from the links around the camera rather than from the inspection itself. The inspection technology overview shows how the parts of a station fit together.
The diagram shows the order of events for one failed product. The gaps between them depend on line speed, processing time and the distance from camera to reject, so they are shown without values.
Swipe sideways to see the whole drawing →
Trigger: telling the camera a product has arrived
Most stations use a photo-sensor mounted just before the camera. When the product breaks the beam, a hardware trigger goes to the camera or vision controller, and the light is strobed or switched on for the exposure. The sensor must see every product once and only once: gaps between products, transparent containers and wobbling products are the usual causes of missed or double triggers.
An encoder on the conveyor gives position instead of time. It is used when speed varies, when products are close together, or when a line-scan camera builds the image line by line as the product moves. The same encoder is often used for reject tracking.
A short exposure keeps a moving product sharp; how short depends on speed and the smallest detail to be seen, and is set during the sample test.
Handing over the result: digital I/O or network
Digital I/O is the simplest route: the station switches outputs wired to PLC inputs, typically pass/fail, a "result valid" pulse that tells the PLC when to read it, ready, and alarm. It is fast, easy to test with a meter and independent of network settings. Output type (NPN or PNP) must match the PLC input card.
An industrial network carries more: the codes read, measurements, the active recipe, counters and a sequence number for each product. It needs the right protocol on both sides.
Protocol support depends on the camera or controller model, not on the software brand as a whole. For the platform we use, HIKROBOT's catalogue states that VisionMaster supports TCP/IP, Modbus, serial port, UDP, EtherNet/IP and other common industrial protocols. Its smart code readers list PLC protocols per model; the IDH7000 row, for example, lists SmartSDK, TCP, FTP, PROFINET, EtherNet/IP, Modbus, UDP and serial, while other reader rows list MELSEC/SLMP and Fins (HIKROBOT Machine Vision Smart Product Catalog, 2025 Q4). PROFINET is therefore listed on specific devices, not everywhere. We check the exact model against your PLC before quoting; where the protocol does not match, a gateway or hard-wired I/O is an option. addaScan builds stations on HIKROBOT VisionMaster; this website does not claim any official HIKROBOT partnership or distributor status.
Tracking the failed product to the reject
The camera and the reject are usually some distance apart, and other products pass in between. The PLC must fire the reject on the right product.
Time delay. The PLC starts a timer when the fail result arrives and fires the reject when it expires. This works on constant-speed lines where products do not slip. It goes wrong when the line slows, stops and restarts, or when products move at different speeds.
Encoder-based tracking. The PLC records the encoder position of each inspected product in a shift register (a queue of positions and results). When the conveyor has moved the product to the reject, the PLC fires. Because tracking follows distance, not time, it stays correct through speed changes and stops.
Two checks keep tracking honest: count triggers against results, so a missing result is noticed; and keep products from accumulating or changing order between camera and reject.
Confirming the reject
Firing the reject is not proof the product left. A confirmation sensor at the chute, in the bin, or just after the reject on the conveyor checks that the failed product left within an agreed window. If the product is still on the line, or a good product was removed, the PLC raises an alarm or stops the line. A bin-full sensor and an air-pressure switch complete the picture.
Designing for faults
What the line does when something goes wrong is a design decision, agreed with your production and quality teams and tested at FAT and SAT. The questions we work through include:
- No result in time: is the product rejected, or the line stopped?
- Camera or controller offline: how does the PLC know (heartbeat, ready signal), and does it stop the line, reject everything, or alarm?
- Air pressure low: the reject may not work; should the line keep running?
- Confirmation failed or bin full: alarm, stop, or both?
- Trigger without a product, or a product without a trigger: how is each detected?
- Inspection bypass: who may switch it off, how is it shown and how is it logged?
This is general control practice. It is not a certified machine-safety function; where a fault could harm people, that is handled by the machine's safety system and its risk assessment.
Selecting the recipe from the PLC
Each product has a recipe: its references, limits and expected codes. At changeover the PLC sends the recipe number or SKU, the station switches and reports back the recipe it is running, and the PLC checks they match before production restarts. Selection on the HMI also works, with access control. A code read from the product can cross-check that the right product is on the line.
Interface checklist
We agree these items in writing before the design is finished. See how we deliver a project for how FAT and SAT are run.
| Item | What to agree | Confirmed during |
|---|---|---|
| Trigger source | Photo-sensor or encoder position; where it is mounted; one trigger per product. | Site survey |
| Encoder | Whether one is fitted or needed, and who reads it: camera, PLC or both. | Site survey |
| Result signals | Pass/fail, result valid, ready, alarm; output type matched to the PLC inputs. | Design review |
| Network | PLC make and model, protocol, and whether the chosen camera or controller lists it. | Design review |
| Heartbeat | How the PLC detects that the station has stopped responding. | Design review |
| Tracking | Time delay or encoder-based shift register; distance from camera to reject. | Design review |
| Reject actuator | Air jet, pusher or diverter; air supply and pressure monitoring. | Site survey |
| Confirmation | Sensor position, confirmation window, bin-full detection. | Design review |
| Fault responses | Agreed action for no result, station offline, low air, failed confirmation, bin full. | FAT and SAT |
| Recipe selection | How the PLC or HMI selects the product, and how the station confirms it. | FAT |
| Data | Which results, images and counters are kept, where, and for how long. | Design review |
| Responsibilities | Who changes the PLC program, who wires the panel, who signs off each test. | Quotation |